When particle physics is a closed subject which has been condensed into a text book, the material will surely be organized by concepts and not according to what fact was learned on what accelerator. But short of that day facilities must be designed, planned, and developed, and experiments must be executed on one of a number of available accelerators; and a very necessary point of view is to ask what physics can be done with one facility, in contrast to another. It is in this spirit that, in this note, we look at electron colliding beam devices. In the first section we discuss the physics that can be done with colliding electron beams. After some general remarks we review the experiments already performed, and then turn to experiments planned for the future. The physics that can be done with any accelerator is a strong function of the physics of the accelerator. Every reader of this Journal knows what determines the energy of an accelerator, but the physics that determines the beam intensity, quality, and pulse length is perhaps not so well known. In fact, we plan to devote a future Comment to the physics that limits the performance of conventional …